Regarding a fuel used in an internal combustion engine included in a combustion system, the fuel supplied to a user has various characteristics, and an index indicating one of the characteristics is a cetane number that is equivalent to an ignitionability. When the fuel that the cetane number is low is supplied, the ignitionability is deteriorated. Therefore, a control object of the combustion system such as an injection time point of the fuel, an injection amount of the fuel, an injection pressure, and an EGR amount, is changed to be easily ignited.
According to JP2013-24138A, the cetane number is correlative to a fuel density. The fuel density is detected to estimate the cetane number, and the control object is changed according to an estimation result. Therefore, a non-combustion HC, a NOx, and a particulate matter (PM) which are included in an exhaust gas can be reduced, and a fuel consumption can be improved.